One of the main ingredients in a cocktail is time. Yes, you want excellent ingredients and fresh citrus and the best liquor appropriate to the drink. 
But if everyone is sitting around enjoying only the taste of their own saliva as they await preparation of your masterpiece, no matter how great the drink will be, the fact that there is not a glass of something wonderful in everyone’s hands detracts from the moment and the ultimate prize, wet and adult in nature. 
Even under the best of conditions, the making of a cocktail takes 3 minutes and in some cases, more than 5. That does not seem like a long time, certainly not when compared to the reading of War and Peace, but when all minds are focused in anticipation of the delivery of a much-desired package, and all eyes are darting about with idle small talk filling the air, well, 5 minutes is a monumentally long time. 
The best a host/hostess can do is to get that first drink out of the prep area and into waiting fingers. Every drink thereafter has the luxury of time. But many drinks do not lend themselves to preparation well in advance, nor do you want your first concoction of the evening to be something that says, “Here, take this. We might be able to do something better later.”
You have the opportunity to make a statement and your own wishes will guide the outcome. If you are quite the perfectionist, then early preparation is your best path. Hours before the guests’ arrival, create that special blend of exotic ingredients that will be the much-praised, you hope, centerpiece of your masterpiece. Fresh fruit and citrus along with spices will keep for a very long time in the refrigerator. They likely will even change, maybe for the better, the day after you have prepared them. 
Hold off on the addition of bitters and the alcohol until just before you serve. In many cases, depending on the mood you wish to set, you may even consider better quality prepared mixes. Not for every drink, of course. Some Bloody Mary pre-mixes are quite good. Zing-Zang has become a local favorite, and Master of Mixes just released a Chef series of Bloody Mary bases. These include Classic, 5 Pepper, and Loaded, boasting “everything but the kitchen sink,” which sounds like a good ingredient to leave out. 
I’ve not had the opportunity to try it, but saw a Bloody Mary mix in the grocery store the other day, Louisiana Sisters, which is from right around here in New Orleans. There are a number of products from this enterprise, including a spicy olive oil ready to make the dirtiest martini of your life. 
When it comes to Margaritas, I am of quite the opposite mind to Bloody Marys. Mixes are not necessary, and they can be a hindrance to a good drink. Margaritas are the simplest of three-ingredient cocktails. I like equal parts fresh-squeezed limes, a good Silver-grade tequila, and Cointreau. You may not like yours so tart, although the Margarita is a citrus-centric cocktail by definition, and maybe you will back off the limes just a bit. But don’t bring the store mixes to the party. There is a lot of sugar in those bottles, and you will be getting all the sugar you need from the Cointreau, plus the bonus of a touch more alcohol. 
Remember the whole idea is for you, too, to be a part of the party. If you are using a loud blender, or have your back to the group as you dilly-dally at the sink, or are constantly sticking your head in the refrigerator, what’s the fun of that for you or your guests? 
The easier you make the preparation while your guests are licking their lips, the more pleasant what can be an awkward moment. Show off how clever you are with jaunty banter, not 8-ingredient cocktails. Have fun!